The present disclosure relates to a correction method, a correction apparatus, and an inspection apparatus. For example, the present disclosure relates to a correction method, a correction apparatus, and an inspection apparatus for correcting unevenness of brightness of a light source in image data used for an inspection of an EUV mask.
For example, in an inspection of a mask for EUV (Extra Ultra Violet) lithography (hereinafter, referred to as an EUV mask) critical illumination is used to ensure brightness of illumination light. The critical illumination is a method for illuminating an EUV mask in such a manner that an image of a light source is formed on an upper surface of the EUV mask, and uses an optical system that can illuminate the EUV mask with high brightness.
In an inspection using critical illumination, a shading correction for correcting a brightness distribution of illumination light in which unevenness of brightness (hereinafter referred to as “brightness unevenness”) (i.e., shading) of a light source is taken into account is performed. For example, when a TDI (Time Delay Integration) detector is used as a detector for an inspection, a shading correction amount is determined based on a brightness distribution of illumination light that is acquired before starting the inspection. Then, a shading correction is performed for each output of the detector.
However, the present inventors have found the following problem. The brightness unevenness of the light source changes with the lapse of time and as a result of this change, the brightness distribution of the illumination light also changes with the lapse of time. If the brightness distribution of the illumination light changes with the lapse of time, a deviation from the shading correction amount acquired before the start of the inspection occurs and it eventually deteriorates the accuracy of the inspection. It is considered that there are two types of brightness unevenness, i.e., brightness unevenness caused by changes in the shape of the brightness distribution and brightness unevenness caused by changes in the position of the light source.
FIGS. 16A to 16C are diagrams showing examples of brightness distributions of illumination light, and show a case where the brightness distribution of the illumination light does not change with time. FIGS. 16D to 16F are diagrams showing examples of brightness distributions of illumination light, and show a case where the brightness distribution of the illumination light changes with time. FIGS. 16A, 16B and 16C show brightness distributions at detection times T0 (t=0), T1 (t=Δt) and T2 (t=2·Δt), respectively. Further, FIGS. 16D, 16E and 16F also show brightness distributions at detection times T0 (t=0), T1 (t=Δt) and T2 (t=2·Δt), respectively.
In each of FIGS. 16A to 16F, the brightness is divided into five levels 1 to 5. An area having a brightness level 5 is an area having the highest brightness. A scanning direction of a detector that detects the brightness distribution of the illumination light is one direction in each of FIGS. 16A to 16F. For example, the scanning direction is a direction D from the bottom of the figure toward the top there of. For example, when the detection times T0 (t=0), T1 (t=Δt) and T2 (t=2·Δt) have elapsed, a detection position U at which the detector performs detection moves along the direction D. For example, as the detection time T elapses, the detection position U changes to positions U0, U1 and U2 on a straight line that extends in the direction D and passes through the center of the brightness distribution.
As shown in FIGS. 16A to 16C, when the brightness distribution of the illumination light does not change over the time T0 (t=0) to T2 (t=2·Δt), no deviation occurs between the shading correction amount acquired before the start of the inspection and the brightness distribution detected at the direction time T. The brightness detected by the detector at the detection positions U0, U1 and U2 are 1, 5 and 1, respectively.
In contrast, as shown in shown in FIGS. 16D to 16F, when the brightness distribution of the illumination light changes over the time T0 (t=0) to T2 (t=2·Δt), a deviation occurs between the shading correction amount acquired before the start of the inspection and the brightness distribution detected at the direction time T. The brightness detected by the detector at the detection positions U0, U1 and U2 are 1, 3 and 5, respectively. Therefore, if the shading correction amount is determined based on the brightness distribution of the illumination light that is acquired before the start of the inspection, the accuracy of the inspection is deteriorated because the brightness acquired before the start of the inspection differs from the actual brightness during the inspection.
As described above, the brightness unevenness of the light source changes with time. Therefore, in the case where the brightness distribution of the illumination light changes with time, it is necessary to determine the shading correction amount for each output of the detector and perform a correction using the determined shading correction amount for each output of the detector. Further, in the case where the power of the light source changes with time, it is also necessary to perform a correction for each output of the detector.
The present disclosure has been made to solve the above-described problem and an object thereof is to provide a correction method, a correction apparatus, and an inspection apparatus capable of accurately correcting temporal variations in a brightness distribution of illumination light and thereby improving accuracy of an inspection.